More particularly the invention relates to the inclusion in a turbine system of the kind described hereinbefore, of means whereby during operation of a said system, hot gas flow through the system and a cooling airflow within the system is such as to provide substantial equalising of the static pressures of the respective flows, in areas where they may mix in an undesirable manner.
Prior art turbine systems are known to provide a cooling airflow to turbine blades in a turbine, so that on take-off of an associated aircraft when maximum temperatures are generated for a short period, the blade mechanical integrity is not affected by the heat.
Take-off normally occupies only a very small proportion of the total flight time of an aircraft. For the majority of its flying time, the aircraft is under cruise conditions in which its associated engines operate at reduced throttle settings. Under these conditions, the gas temperatures within the engines reduce as do the stresses imposed upon the components of the engines as engine rotational speed reduces. Consequently, under these cruise conditions, the cooling airflow to the engine's turbine blades is usually in excess to that which is actually required. The excess cooling air leaves the blades and enters the gas flow where it mixes and reduces the gas temperature and pressure. The thermal efficiency of the system, and therefore the specific fuel consumption of the engine, are thereby degraded.
Our co-pending Patent Application U.S. application Ser. No. 08/953,006 entitled `Method and Apparatus for Controlling Cooling Air in a Gas Turbine Engine` filed at the same time as the application accompanying this specification provides an improved manner and apparatus for reducing the cooling airflow over and above the reduction thereof achieved by reduced compressor output by providing valve means in the airflow path adjacent its entry into the turbine system. The valve means is operable to enable full compressor cooling air output to be varied, to the point of all but completely stopping the flow from entering the turbine system.